Field of the Invention
The present invention relates to an aberration computing device, aberration computing method, image processor, image processing method, and electron microscope.
Description of Related Art
A diffractogram indicative of Fourier transformed intensities of an amorphous image is used as a means for detecting defocus and two-fold astigmatism in transmission electron microscopy. A technique for quantitatively measuring defocus and two-fold astigmatism based on the diffractogram is important in making aberration corrections and in other similar applications.
For example, J. Barthel, A. Thust, Ultramicroscopy 111, pp. 27-46 (2010) discloses a technique for finding defocus and two-fold astigmatism by two-dimensional pattern fitting of a diffractogram. In particular, in preprocessing a diffractogram, the background is removed and an envelope function is complemented. Then, the binarized diffractogram is compared against a database, thus roughly measuring defocus and two-fold astigmatism. Then, pattern fitting of experimental data about the diffractogram to simulation results is done by using the covariance on a serpentine path as a degree of similarity. Values of the defocus and two-fold astigmatism are found by performing the pattern fitting while varying three parameters of the defocus and two-fold astigmatism (x and y).
M. Vulovic et al., Ultramicroscopy 116, pp. 115-134 (2012) discloses a technique of finding defocus and two-fold astigmatism by template matching. In particular, the S/N is improved by periodgram averaging as a pretreatment. Then, the background is reduced by taking a logarithm of the signal. Furthermore, the intensity is inverted by multiplying the signal by a second-order Gaussian differential filter. After the diffractogram is transformed into polar coordinates, the coordinates are transformed into a three-dimensional parameter space using template matching and a maximum value is detected. By finding the position of a maximum value in this way and plotting the ellipticity relative to the spatial frequency, defocus and two-fold astigmatism are measured.
In the technique of the above-cited J. Barthel, A. Thust, Ultramicroscopy 111, pp. 27-46 (2010), however, two-dimensional pattern fitting is applied to a diffractogram. In the technique of the above-cited M. Vulovic et al., Ultramicroscopy 116, pp. 115-134 (2012), a diffractogram is transformed into polar coordinates and then into a three-dimensional space. In this way, the techniques of J. Barthel, A. Thust, Ultramicroscopy 111, pp. 27-46 (2010) and M. Vulovic et al., Ultramicroscopy 116, pp. 115-134 (2012) involve computations in higher dimensions and so there is the problem that the computational speed is low, especially where the image size is large.